Le Chatelier’s Principle Explained Simply

Pressure only matters for gaseous systems.

Increasing pressure

Equilibrium shifts toward the side with fewer moles of gas.

Decreasing pressure

Equilibrium shifts toward the side with more moles of gas.

Changing volume

Has the opposite effect of pressure changes
(Decreasing volume increases pressure; increasing volume decreases pressure.)

Example:
N₂ + 3H₂ ⇌ 2NH₃
Left side: 4 moles of gas
Right side: 2 moles of gas
Increasing pressure shifts the equilibrium right.

Temperature and Exothermic vs. Endothermic Reactions

Temperature shifts depend on reaction enthalpy.

Exothermic reaction (ΔH < 0)

Heat is a "product".
Increasing temperature → system shifts left.
Decreasing temperature → system shifts right.

Endothermic reaction (ΔH > 0)

Heat is a "reactant".
Increasing temperature → system shifts right.
Decreasing temperature → system shifts left.

This is a crucial point frequently tested in IB exams.

Catalysts and Equilibrium Position

A catalyst:

• Speeds up forward and reverse reactions equally
• Helps the system reach equilibrium faster
• Does not shift equilibrium position

Le Chatelier’s Principle does not apply to catalysts.

Why Le Chatelier’s Principle Works

The principle reflects the system’s tendency to restore balance.
At equilibrium, the forward and reverse reaction rates are equal.
When a disturbance occurs, the system adjusts one rate relative to the other until balance is restored.

This is a dynamic process—molecules are constantly reacting even at equilibrium.

IB Exam Tips for Applying the Principle

• Identify the change first (concentration? temperature? pressure?)
• Determine whether heat is a reactant or product
• Count moles of gas, not solids or liquids
• Write arrows to visualize shifts (← or →)
• Remember: catalysts do nothing to position
• Discuss rate first, then position when explaining dynamic equilibrium

This structured approach prevents common errors.

Example (IB-Style)

Given the equilibrium:
2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = –196 kJ/mol

Predict the shift if temperature increases.

Because ΔH is negative, the reaction is exothermic.
Adding heat pushes the equilibrium left, favoring reactants.

FAQs

Does equilibrium mean concentrations are equal?

No. Equilibrium means the rates are equal. Concentrations remain constant but not necessarily equal.

Does Le Chatelier’s Principle predict reaction rate changes?

Only indirectly. It mainly predicts direction of shift, not how fast equilibrium is reached.

Do solids and liquids affect equilibrium shifts?

Not in concentration changes. Only aqueous and gaseous species count for concentration shifts.

Conclusion

Le Chatelier’s Principle predicts how an equilibrium system responds to changes in temperature, pressure, or concentration. By understanding whether a shift increases or decreases a disturbance, you can confidently analyze equilibrium behavior in real reactions and IB exam questions. Mastering this principle is essential for thermodynamics, equilibria, and chemical reasoning.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.